Printed circuit boards have a large number of holes which must be drilled at precise locations to enable mounting of electrical components and to make connections between circuits on different layers. Computer-controlled drilling machines have been developed to permit mass production of circuit boards at high rates. Typically, these drilling machines have a worktable mounted on a base and movable along both axes of a horizontal plane. The printed circuit boards, or workpieces, are mounted in stacks on the worktable. Movement of the worktable in the horizontal plane positions the circuit boards beneath a drill spindle at the precise location where holes are desired. Hole drilling occurs when the spindle moves downward.
Since the diameter of the holes drilled in the circuit boards can be extremely small, on the order of the size of human hairs, and the drilling tools are delicate, it is imperative that movement of the spindle be rapid, precisely controlled and that extraneous forces and moments do not deflect the rotational axis of the spindle from the intended location and direction of the hole. In prior systems, the vertical position of the spindle is controlled by a motor-driven leadscrew.
Several drawbacks are prevalent in these prior systems such as that illustrated in FIG. 1.
The moment of inertia of the leadscrew 1, attached directly to the motor shaft 2, impedes the ability of the motor 3 to accelerate the movable element 11.
Unless the center of gravity 5 of the spindle lies on the leadscrew axis 6, inertial reactions to leadscrew 1 forces produce moments that tend to deflect the spindle rotational axis 7 from the intended location of the hole.
Placing a measurement device for control-system feedback, such as a linear scale, in line with the spindle center of gravity 5 minimizes position errors arising from minute angular deflections of the spindle 4. Because both leadscrew 1 and linear scale cannot occupy the same space, use of a leadscrew 1 forces a design compromise that increases position errors.
The motor 3 is attached to the leadscrew by means of a flexible coupling 8 to prevent minor misalignment of the motor 3 and leadscrew 1 from overloading the motor 3 or leadscrew 1 bearings. This coupling 8 is costly, is subject to fatigue and breakage, and adds to the moment of inertia of the motor load.
A pair 9 of ball thrust bearings that are heavily pre-loaded against each other restrains axial motion of the leadscrew 1. These bearings 9 are costly, add to the motor's frictional load, and become loose near the end of their wear life, allowing uncertainties in the spindle 4 axial position.
The nut housing 10 that moves vertically up and down the leadscrew 1 is attached to the spindle housing 11 by means of thin struts 12 that flex to accommodate minor misalignments of the leadscrew 1 and the spindle's guideway 13. These struts 12 and their means of mounting add to the complexity and cost of the assembly.
Because the amount of misalignment tolerated by the flexible coupling 8 and struts 12 is very small, and because the mountings for the motor 3, the thrust bearings 9 and flexible struts 12 must be carefully aligned to the guideway 13, they permit only very small machining tolerances.
The leadscrew 1 must be ground to very small tolerances to avoid position errors arising from axial play and friction. Looseness of the nut 14 on the leadscrew 1 near the end of its wear life gives rise to uncertainty of spindle axial position during drilling.
The tendency of the nut 14 and its housing 10 to rotate must be resisted by a roller 25 that runs in a slot 26. This roller is costly and subject to wear.
Thus, a need exists for a linear motion-control system in which the inertia load on the motor is less, in which the cost and inertia of flexible couplings can be avoided, in which the cost and wear of guide rollers can be avoided, in which the machining tolerances are less critical, in which a measuring device can be mounted on the spindle rotational axis, and in which position uncertainty does not occur near the end of wear life.